Magnetoresistive (MR) angle sensors typically are thin, flat structures formed on a main surface of a substrate, such as a semiconductor die. The projection of a magnetic field onto this main surface is referred to as the in-plane magnetic field. MR sensors directly measure the angle between the in-plane magnetic field and a reference direction in the main surface. MR sensors generally do not, however, measure the magnitude of the magnetic field. In many applications, this is a drawback. For example, MR angle sensors typically comprise a small permanent magnet that is attached to the shaft and whose position is to be measured. This magnet, however, can become unattached, chipped or broken, or can attract a loose piece of metal that shorts a portion of the magnetic field. These and other situations can provide angle measurement errors that often cannot be detected. It can also be beneficial to monitor the strength of the magnet over time to detect lifetime drift or corrosion or to detect whether strong magnetic fields from nearby machines and systems are acting on the sensor.
One particular type of MR sensor is an anisotropic MR sensor, or AMR sensor. AMR sensors are often less expensive, more accurate and more robust than other sensors. AMR sensors can also measure magnetic field angle but only between 0 degrees and 180 degrees. For some applications, such as steering angle position sensing, it is desired to measure an entire resolution, from 0 degrees to 360 degrees.
MR sensors are also subject to hysteresis due at least in part to impurities in the MR layers, which means that the output signal lags the true angle of the applied magnetic field by about 0.1 degrees to about 1 degree. This is another drawback associated with conventional MR sensors.
Therefore, there is a need for an improved MR sensor.